Diabetes mellitus, or diabetes, is an illness in which there is an abnormally high level of glucose in the blood. Depending on how high your glucose level is and how long it has been high, you may feel fairly well, or you may be so sick that you require hospitalization. Usually, your doctor will test you for diabetes if you have symptoms such as thirst, frequent urination, weight loss, blurred vision, and fatigue.

This article helps you understand how diabetes is defined and classified and how physicians test for the disease.

Glucose is a sugar and is one of the energy sources of the body. Some organs in our bodies, such as the brain, are particularly dependent upon glucose as an energy source, so it is very important that the body maintain the amount of glucose in the blood in the normal range: if the level is too high or too low, there are serious consequences.

To avoid these consequences, the body has a complex set of mechanisms to keep the glucose in the normal range.

The liver is in charge of taking up and releasing glucose into the bloodstrea. After a meal, the blood carrying nutrients from digestion first flows through the liver, which removes the excess glucose. When the glucose level in the blood drops (for example, after fasting or exercising), the liver does the opposite and releases glucose into the bloodstream. The liver knows how to regulate the level of glucose in the blood because it receives signals from hormones, which are chemical  messengers in the blood. The two hormones that are particularly important in diabetes are insulin and glucagon. These hormones are produced in the islets of Langerhans of the pancreas, an elongated organ located behind and below the stomach in the abdomen. There are about a million islets in a normal pancreas, and they consist of several types of cells the beta cells make insulin and the alpha cells make glucagon In a person with diabetes, the beta cells in the islets fail, and this alters the balance of insulin and glucagon actions on the tissues. The cause and degree of beta cell failure varies in different kinds of diabetes.